The objective compound of this invention (III) is already known e.g. by the specification of Japanese Patent Applications Kokai Nos. 51-68,568 and 52-33,692. However, when the examples of said patent literatures were traced by the present invention, the yield was found to be about 20 to 30%. They then sought several measures for improving the yield to find that when the hydroxyl group of the hydroxyethyldithiocarbamate is protected in an ether form, the yield was raised up to 90%. Based on this discovery, this invention was established.